[processing] Correctly expose full expression context to Filter Features algorithm

configuration widget in modeler

Allows filters to be built (in the GUI) using context related variables
and functions such as input parameter values, model variables, etc

Author: nyalldawson

src/gui/processing/qgsprocessingconfigurationwidgets.cpp

@@ -100,6 +100,7 @@ void QgsFilterAlgorithmConfigurationWidget::setConfiguration( const QVariantMap
     mOutputExpressionWidget->insertRow( currentRow );
     mOutputExpressionWidget->setItem( currentRow, 0, new QTableWidgetItem( output.value( "name" ).toString() ) );
     QgsExpressionLineEdit *expressionBuilder = new QgsExpressionLineEdit();
+    expressionBuilder->registerExpressionContextGenerator( this );
     expressionBuilder->setExpression( output.value( "expression" ).toString() );
     mOutputExpressionWidget->setCellWidget( currentRow, 1, expressionBuilder );
     QCheckBox *isModelOutput = new QCheckBox();
@@ -143,6 +144,7 @@ void QgsFilterAlgorithmConfigurationWidget::addOutput()
   int rowIndex = mOutputExpressionWidget->rowCount();
   mOutputExpressionWidget->setRowCount( rowIndex + 1 );
   QgsExpressionLineEdit *expressionBuilder = new QgsExpressionLineEdit();
+  expressionBuilder->registerExpressionContextGenerator( this );
   mOutputExpressionWidget->setItem( rowIndex, 0, new QTableWidgetItem( QString() ) );
   mOutputExpressionWidget->setCellWidget( rowIndex, 1, expressionBuilder );
   mOutputExpressionWidget->setCellWidget( rowIndex, 2, new QCheckBox() );